Bugfix: cleanup didn't unswizzle classes that had already been swizzled
Bugfix: cleanup didn't unswizzle classes that had already been swizzled

This would happen when using other code that itself swizzles classes (i.e. ReactiveCocoa). For example, if a rac_signalForSelector was called on an object then later an aspect hook was installed and further removed, we would end up in a situation where Aspect's class swizzling (in-place swizzling of -forwardInvocation:) would never be ununstalled when the last hook was removed, later wrecking havoc with RAC's own use of -forwardInvocation.

In short, this pull request fixes hair-pulling crashes.

21451061/Aspects,HelloZhu/Aspects,yaoxiaoyong/Aspects,steipete/Aspects,bing6/Aspects,ReutersTV/Aspects,06170728/Aspects,FelixMLians/Aspects,sunfei/Aspects,DevSonw/Aspects,RyanTech/Aspects,Jeff-F/Aspects,bekin/Aspects,guiquanz/Aspects,fanyingzhao/Aspects,hanhailong/Aspects,rainkong/Aspects,KBvsMJ/Aspects,litianming/Aspects,vincentiss/Aspects,iOS--wsl--victor/Aspects,morizotter/Aspects,yangchao0033/Aspects,z8927623/Aspects,policp/Aspects,gaolanxu93/Aspects,Anish-kumar-dev/Aspects,zhouwude/Aspects,mr-v/Aspects,liuchungui/Aspects,common2015/Aspects,bufanwyl/Aspects,keslcod/Aspects,joywt/Aspects,tranhieutt/Aspects,ccguo/Aspects